Exceptions in the periodic table

A list of Periodic table exceptions include

  • Sizes of inert gases, transition metals, and inner transition metals at the atomic level.
  • The first element of a group behaves abnormally.
  • Stability of oxidation within the p block
  • When the orbitals are half-filled or filled, the ionization energy is unusually high.

Changes in ionization enthalpies

Along the period, as the number of atoms gets higher, so does the ionization enthalpy. Some elements don’t follow this pattern because of the type of electron to be removed.

The extra stability of electronic configurations that are exactly half-filled or filled.

Changes along the period

In the second period, from Li to Be, the ionization enthalpy keeps rising because the atomic radius of Be is smaller than that of Li and the nuclear charge is higher.

For Be to B, even though B has a higher nuclear charge than Be, B has a lower ionization enthalpy than Be.

From B to C to N, these elements’ first ionization enthalpies keep going up because their nuclear charges keep going up and their atomic sizes keep going down. For N to O, the first ionization energy of oxygen is less than that of nitrogen.

Even though oxygen has a higher nuclear charge than nitrogen. For O to F to Ne, the first ionization enthalpy goes up as the nuclear charge goes up from O to F to Ne.

  • Similar changes can be seen in the first ionization enthalpies of the elements in the third period.

Changes in the enthalpies of electron gain in a group

As we move down a group, electron gain enthalpies tend to get more negative. The electron gain enthalpies of some second-period elements.

Like F and O, are less negative than those of their third-period counterparts, like S and Cl. So, F has less energy to gain an electron than Cl.

  • Also, noble gases have a positive electron gain enthalpy because the subshells of their atoms are all full.
  • From halogen (F) to an inert gas, the atomic radius changes (Ne). Since all of the orbitals are fill in an inert gas, the electronic repulsions are at their greatest.
  • As we move across a period, the number of electrons with valence goes from 1 on the left to 8 on the right. When it comes to H or O, an element’s valence first goes up to 4, then down.

Exceptions in the periodic table of some elements


Mercury is the only metal in the periodic table that is a liquid at room temperature. The ancient Greeks used mercury as a cosmetic and to make alloys with other metals. It has the strange property of being a liquid.

In contrast to other metals, Mercury is hard to share electrons with due to its structure. Due to this, it has a low melting point and doesn’t conduct electricity well, unlike other metals.


Group 7 of the periodic table contains another liquid. The only other known liquid in the periodic table is bromine.  Chlorine exists as a gas, Fluorine exists as a liquid, and Iodine as a solid.

As a result, the simple diatomic bromine molecule Br2 does not become a gas at room temperature because it has such strong intermolecular interactions.

Bromine molecules will escape from the surface of the bromine liquid in a sealed container. Resulting in the presence of a brownish-red gas.


Carbon is considered to have been a gas, like its neighbor’s Oxygen and Nitrogen. The most well-known chemical of Carbon Dioxide exists as a gas. Carbon, but, exists in allotropes.

Allotropes are the various physical forms that an element can take. Carbon is an outstanding example of this. Diamond and Graphite are the most prevalent types of Carbon.

As carbon possesses these two covalent structures with high melting points, it is classified as a solid.

Other exceptions in the periodic table

Alkali metal is listed before noble gas in the periodic table because of its lower atomic weight.  Alkali metals follow immediately after noble gases (elemental weights are a little higher).

The periodic chart would have an odd anomaly if we didn’t make an exception for Ar and K’s growing atomic weights. Potassium, a reactive silver metal, is one of the elements in the vertical column of noble gases (K).

Gases such as argon, which are neither metals nor particularly reactive, belong to the group of reactive alkali metals.

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